1. Field of the Invention
The present invention relates to an optical writing device or an optical scanning device and an optical writing method used in an image forming apparatus such as a copier, a printer, a facsimile machine, and a multifunction machine including at least two of these functions.
2. Discussion of the Background Art
In the market for image forming apparatuses, needs have arisen to reduce the size, the weight, and the cost of the apparatus. In particular, color image forming apparatuses include more components and are larger than typical monochrome image forming apparatuses. Therefore, more compact color image forming apparatuses are highly desired.
A commercially available image forming apparatus includes an optical writing device (an optical scanning device) that scans surfaces of image carriers with multiple light beams, respectively, by reflecting each light beam multiple times by multiple reflecting mirrors. The use of multiple mirrors, however, makes the optical writing device thicker.
FIG. 1 illustrates an example of a configuration of a known optical writing device. In the optical writing device shown in FIG. 1, light beams emitted from light sources, not illustrated, are deflected by a polygon mirror 54 and then reflected by multiple mirrors (reflecting mirrors) 59 to be respectively guided to photoconductors 101, which are targets to be scanned. The mirrors 59 for guiding the light beams to the photoconductors 101 should be arranged such that the light beams do not interfere with one another. Such arrangement naturally requires a larger space. In particular, the height of the optical writing device is increased.
In order to achieve the desired compactness, several approaches are known. For example, one known technique provides an optical scanning device for a multi-color image forming apparatus that scans multiple photoconductors using two light sources whose deflection directions or wavelengths are different from each other. Another known technique provides an optical scanning device and an electrophotographic recording device that use a multi-chip semiconductor laser light source including multiple juxtaposed semiconductor laser chips that emit laser beams of different wavelengths so as to scan multiple photoconductor drums separately. Yet another known technique provides an image forming apparatus that includes a separator that directs multiple deflected laser beams in different directions to different areas of a target object in image forming operation.
In the approaches described above, light beams are synthesized and then separated by the separator. However, lengths of the optical paths from the light sources to the photoconductors may be different. Alternatively, even when the optical path lengths are equalized, a scanning surface of the polygon mirror, which is a surface formed with the light beam directed by the polygon mirror, cannot parallel a plane that connects positions of the multiple photoconductors onto which the light beams are respectively directed. (Hereinafter, the position onto which the light beam is directed is referred to as a scanned position.)
As a result, the light beams may reach on the photoconductors at different angles of incidence. Hereinafter the position onto which the light beam is directed is referred to as a scanned position.
In arrangement in which the scanning surface of the polygon mirror does not parallel the plane connecting the scanned positions on the respective photoconductors, the overall size of the optical writing device can be reduced. Still, the thickness of the optical writing device cannot be reduced because the multiple reflecting mirrors are used therein, and thus the image forming apparatus remains relatively large.
Further, in a four-color optical writing device in which optical devices are located substantially symmetrically across the polygon mirror, it is difficult to arrange the photoconductors in a line. Therefore, such an optical writing device cannot be used in a tandem color image forming apparatus, which is currently widely used.
By contrast, in other examples in which the optical path lengths from the light source to the photoconductors and the angles of incidence of the light beams on the photoconductors are different from each another, diameters of the light beams on the surfaces of the photoconductors may be different from each other as well, which is undesirable. Further, such a configuration is disadvantageous when attempting to superimpose different color images formed on the photoconductors.